Rock felling in a mine or any other type of underground excavation is usually performed by blasting up with explosive charges, and is followed by an extraction phase during which the thus felled rocks are removed from the working site. Miners must thereafter purge the excavation walls, i.e. generate the fall of rock blocks that have a tendency to detach therefrom, for example with a scaling bolt. After that, the stability of the thus formed shaft walls (of the tunnel, or of the chamber), must be confirmed by implementing one or more supporting techniques. The support of rocky bed is paramount for the workers' safety and for the effective operations in the mine.
There are various supporting techniques, one of which being use of anchoring bolts, for stabilising the rocky bed. So-called mechanical anchoring bolts, or “rockbolt”, are the most commonly used in view of their low cost and ease and quickness of installation. A rockbolt comprises an elongated threaded rod, also called stud bolt, at the distal end of which is mounted an expansion shell of generally cylindroid shape. The expansion shell includes a number of elongated metallic blades (typically 2 or 4) surrounding the stud bolt, all-interconnected to one another by one of their ends, and whose external surface is toothed and thus rough surfaced. As with any other anchoring bolt,this type of anchoring bolt is also provided with a bearing plate mounted at the proximal end of the stud bolt. By having the stud bolt rotate in a given direction in relation to the expansion shell, the blades of the expansion shell with open as if petals of a blooming flower, and the expansion shell widens radially.
Before proceeding with the installation of such an anchoring bolt, an elongated bore is drilled orthogonally to the surface of the rock bed wall to stabilise. The bore diameter should be slightly larger than that of the cylindroid expansion shell, to enable ready engagement of the bolt to the bottom of the bore. Thereafter, a worker must rotate the stud bolt by using for example a rotating shaft pneumatic tool. Since the expansion shell cannot freely rotate in the bore because it frictionally abuts against the internal surface of the bore, the rotation of the stud bolt generates a relative rotation of the stud bolt relative to the expansion shell, which enables the latter to open up and enables its toothed blades to firmly grasp the bore wall. Once the expansion shell is anchored in this way in the bore, a nut located at the proximal end of the stud bolt must be screwed against the plate in order to press the latter against the excavation wall, and in order to load the bolt. When the anchoring bolt is loaded, stabilisation of the rocky bed is enabled by the combination action of the pressure exerted by the plate against the excavation wall, and by the pressure exerted by the expansion shell inside the bore.
Such rock bolts have advantages. In particular, these rock bolts are easily and quickly mounted, they are cheap, and they provide the excavation wall with an active support, i.e. they become operational and loaded immediately after their installation. On the other hand, they do have important drawbacks, such that they do not allow their use in all circumstances. In particular, they boast a mediocre performance when used with broken or soft rocks, and are sensitive to vibrations, i.e. they may lose their load following a ground blow or an adjacent blast. Moreover, these bolts have a very small resistance to shear forces. Indeed, if the shear forces are too high at the interface between the internal surface of the bore and the toothed blades of the shell, the internal surface of the bore may crumble, and the teeth of these blades may then release their grip from the internal surface of the bore. Moreover, once they lose their load, the effectiveness of these bolts shift instantly from 100% to 0%, since the expansion shell has then lost its grip on the internal surface of the bore. This could be dangerous, since unless each bolt is individually tested to check if it is still loaded, a mounted bolt having lost its load may leave the impression to the workers that it is still operational whereas in fact it has become none at all. Other types of anchoring bolts also exist and are frequently used to support underground excavation walls, for example cemented bolts. A cemented bolt consists essentially of a toothed bolt surrounded by cement and carrying a bearing plate at its proximal end. Installation of a cemented bolt normally starts by the drilling of an elongated bore, in the same way as for a mechanical bolt, followed by thick-paste cement injection inside the bore. Once cement has been injected, the elongated toothed bolt is engaged into the bore until the bearing plate comes to abut against the excavation wall. The bolt is then loaded passively by the convergence of the rock bed. During this convergence (where the excavation walls tend to close on themselves due to the large pressures naturally present in the rocky bed and amplified by the underground excavations), the bearing plate of the bolt will transfer the load to the bolt. The friction between the cement/rock and the cement/rod contacts, contribute in stabilising the excavation wall in which the cemented bolt is mounted.
This type of bolt is very resistant in tension, and may be used in all types of rocks, even in solft and very broken rocks, contrarily to mechanical bolts. Moreover, this type of bolt is much more resistant to shearing forces than mechanical bolts, and do not lose all its load if the shearing forces become excessive, contrarily to mechanical bolts. On the other hand, installation of cemented bolts takes a lot of time and is expensive, requires use of a cement pump, and is messy and thus not user-friendly for the workers. Moreover, installation of such bolts in vertical position, for example on the excavation top wall, is labour intensive, since it requires cement injection in a vertical bore.
Other anchoring devices may be used for the support of excavation walls such as resin bolts, or friction bolts of the “Split Set” type consisting of a slitted steel tube being hammered into the bottom of the drilled bore and into the excavation wall, but all have a large number of drawbacks.